This paper applies data envelopment analysis (DEA) to explore the quantitative relative efficiency of 18 classes of freshmen students studying a course of English conversation in a university of Taiwan from the academic year 2004-2006. A diagram of teaching performance improvement mechanism is designed to identify key performance indicators for evaluation in order to help teachers concentrate their efforts on the formulated teaching suggestions. The sensitivity study highlights the priority of the richness of course contents over the other evaluated indicators. The performance improvement mechanism can help decision-makers to design educational policies. (Contains 2 figures and 7 tables.)